High-Voltage FDS of Thermally Aged XLPE Cable and Its Correlation with Physicochemical Properties

This paper aims to investigate the influence of thermal aging on a crosslinked polyethylene (XLPE) cable, and the relationships between the macroscopical high-voltage dielectric and the microscopical physicochemical properties are also elucidated. To better simulate thermal aging under working condition, the medium-voltage-level cable is subjected to accelerated inner thermal aging for different aging times. Then, high-voltage frequency domain spectroscopy (FDS) (cable sample) and analyses of microscopic physical and chemical properties (sampling from the cable), including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and elongation at the break (EAB), are conducted at different cable aging stages. The dielectric test results show that after a certain aging time, the high-voltage FDS curves of the cable have layered characteristics, and this phenomenon is more obvious as the aging degree increases. Moreover, the slope and the integral of the high-voltage FDS curves rise with aging time. The mechanism is deduced by the physicochemical results that thermo-oxidative aging results in increasing polar groups and dislocation defects in the crystal region, which leads to the above phenomenon. On the one hand, the appearance of polar groups increases the density of the dipole. On the other hand, the destruction of the crystal region increases the probability and amplitude of dipole reversal. In addition, the breaking of molecular bonds and the increase in the amorphous phase also reduce the rigidity of the XLPE molecular main chain. The above factors lead to obvious delamination and larger dielectric parameters of the thermally aged cable. Finally, according to the experimental results, an on-site diagnosis method of cable insulation thermal aging based on high-voltage FDS is discussed.


Introduction
Crosslinked polyethylene (XLPE) is widely used as an electrical insulator in extruded power cables [1]. However, it is now understood that XLPE cables experience aging when subjected to various stresses (electrical, thermal, mechanical, etc.) in service. Among them, the thermal constraint is considered as the most severe factors causing irreversible damage to the electrical insulation [2,3]. The damage strongly reduces the performance of the XLPE cable and its physical morphology, potentially leading to failure.
The morphological changes occur in XLPE due to thermal aging. Many researchers have focused on physicochemical analyses of thermal aging caused by parameter changes. The representative of these are melting point [4], crystallinity [5], infrared carbonyl absorbance [6], oxidation induction time, electrical strength, tensile strength [7], and elongation at break (EAB) [8]. However, using the above properties to monitor the aging states and evaluate the conditions of the cable insulation still has quite a few problems. First of all, some methods require sophisticated equipment, which are expensive and time consuming. Figure 1 shows the XLPE cable sample under pretreatment. The YJV-8.7/10 kV XLPE cable from Yuandong company, Shandong, China (insulation thickness 4.5 mm, 4.2 m) is used for the thermal aging experiment and the subsequent tests. The outer semiconducting layer near the copper joints is peeled off by 5 cm to 10 cm for safety consideration. Copper joints are installed at both ends of the sample for better connection. The rest of this article is organized as follows. Section 2 describes the sample preparation process and experimental methods. The high-voltage FDS and microscopical physicochemical test results are displayed in Section 3, and the mechanism for the changes in high-voltage FDS during the thermal aging process are analyzed. The relationships between the macroscopical high-voltage dielectric and the microscopical physicochemical properties are also elucidated. Furthermore, the possibility and feasibility of using highvoltage FDS as a field test method for cable thermal aging is also discussed in this section. Finally, the conclusions are stated in Section 4. Figure 1 shows the XLPE cable sample under pretreatment. The YJV-8.7/10 kV XLPE cable from Yuandong company, Shandong, China (insulation thickness 4.5 mm, 4.2 m) is used for the thermal aging experiment and the subsequent tests. The outer semiconducting layer near the copper joints is peeled off by 5 cm to 10 cm for safety consideration. Copper joints are installed at both ends of the sample for better connection. The overall thermal aging of cables in operation is generally caused by overcurrent in the conductors. In order to better simulate the overheating condition under an operation situation, the internal heating method is used in the accelerated thermal aging experiment. The overall accelerated thermal aging system (composed of large current generator and temperature monitoring system) is shown in Figure 2. The large current generator is used to heat the cable conductor. In order to control the heating temperature, the temperature acquisition system is used to monitor the temperature of both the cable sample and short temperature measurement sample (60 cm). From Figure 2, ① and ② are temperature-measuring thermocouples. ① accesses to the copper core of the temperature measurement cable sample; ② and ③ measure the temperature of the outer sheath of both the temperature measurement cable and the actual aging sample. According to IEC-60502, the thermal aging treatment temperature of XLPE cable insulation is 135 °C. Thus, the heating temperature is set to be 135 °C [22]. The conductor temperature can be observed at any time through the temperature-measuring thermocouple to determine that the temperature at the most strongly heated part of the cable insulation is 135 °C. In this experiment, the aged cable insulation is cut into the inner, middle, and outer layer strip samples using a crosscutting machine. The 1.5 mm XLPE layer is cut by the crosscutting machine first, then three XLPE strips 0.6 mm in thickness are cut from the remaining 3 mm layer. Finally, the dumbbell samples are formed using a dumbbell die The overall thermal aging of cables in operation is generally caused by overcurrent in the conductors. In order to better simulate the overheating condition under an operation situation, the internal heating method is used in the accelerated thermal aging experiment. The overall accelerated thermal aging system (composed of large current generator and temperature monitoring system) is shown in Figure 2. The large current generator is used to heat the cable conductor. In order to control the heating temperature, the temperature acquisition system is used to monitor the temperature of both the cable sample and short temperature measurement sample (60 cm). From Figure 2, 1 and 2 are temperaturemeasuring thermocouples. 1 accesses to the copper core of the temperature measurement cable sample; 2 and 3 measure the temperature of the outer sheath of both the temperature measurement cable and the actual aging sample. According to IEC-60502, the thermal aging treatment temperature of XLPE cable insulation is 135 • C. Thus, the heating temperature is set to be 135 • C [22]. The conductor temperature can be observed at any time through the temperature-measuring thermocouple to determine that the temperature at the most strongly heated part of the cable insulation is 135 • C.

Sample Preparation
The rest of this article is organized as follows. Section 2 describes the sample preparation process and experimental methods. The high-voltage FDS and microscopical physicochemical test results are displayed in Section 3, and the mechanism for the changes in high-voltage FDS during the thermal aging process are analyzed. The relationships between the macroscopical high-voltage dielectric and the microscopical physicochemical properties are also elucidated. Furthermore, the possibility and feasibility of using highvoltage FDS as a field test method for cable thermal aging is also discussed in this section. Finally, the conclusions are stated in Section 4. Figure 1 shows the XLPE cable sample under pretreatment. The YJV-8.7/10 kV XLPE cable from Yuandong company, Shandong, China (insulation thickness 4.5 mm, 4.2 m) is used for the thermal aging experiment and the subsequent tests. The outer semiconducting layer near the copper joints is peeled off by 5 cm to 10 cm for safety consideration. Copper joints are installed at both ends of the sample for better connection. The overall thermal aging of cables in operation is generally caused by overcurrent in the conductors. In order to better simulate the overheating condition under an operation situation, the internal heating method is used in the accelerated thermal aging experiment. The overall accelerated thermal aging system (composed of large current generator and temperature monitoring system) is shown in Figure 2. The large current generator is used to heat the cable conductor. In order to control the heating temperature, the temperature acquisition system is used to monitor the temperature of both the cable sample and short temperature measurement sample (60 cm). From Figure 2, ① and ② are temperature-measuring thermocouples. ① accesses to the copper core of the temperature measurement cable sample; ② and ③ measure the temperature of the outer sheath of both the temperature measurement cable and the actual aging sample. According to IEC-60502, the thermal aging treatment temperature of XLPE cable insulation is 135 °C. Thus, the heating temperature is set to be 135 °C [22]. The conductor temperature can be observed at any time through the temperature-measuring thermocouple to determine that the temperature at the most strongly heated part of the cable insulation is 135 °C. In this experiment, the aged cable insulation is cut into the inner, middle, and outer layer strip samples using a crosscutting machine. The 1.5 mm XLPE layer is cut by the crosscutting machine first, then three XLPE strips 0.6 mm in thickness are cut from the remaining 3 mm layer. Finally, the dumbbell samples are formed using a dumbbell die In this experiment, the aged cable insulation is cut into the inner, middle, and outer layer strip samples using a crosscutting machine. The 1.5 mm XLPE layer is cut by the crosscutting machine first, then three XLPE strips 0.6 mm in thickness are cut from the remaining 3 mm layer. Finally, the dumbbell samples are formed using a dumbbell die machine for mechanical, FTIR and XRD test. Figure 3 shows the schematic diagram of the cable crosscutting. The samples required for microscopical and physicochemical measurement (FTIR, XRD, and mechanical test) are taken from the most strongly aging part of the cable (inner slice).  Figure 3 shows the schematic diagram cable crosscutting. The samples required for microscopical and physicochemical m ment (FTIR, XRD, and mechanical test) are taken from the most strongly aging pa cable (inner slice). The aging time lasts 1536 h (64 days). The overall thermal aged cable is te high-voltage FDS at the aging time points of 48 h, 96 h, 192 h, 384 h, 768 h, and respectively, and the material is also collected for the mechanical properties tes and XRD at the same time points.

Macroscopic High-Voltage FDS Measurement
The high-voltage FDS testing platform is composed of a high-voltage FDS d system and cable sample, as shown in Figure 4. The cable core is connected to the end of the detection system, and the cable ground wire is connected to the curren tion end. The FDS is measured in the following order: start at 0.25 U0, then incr intervals of 0.25 U0, gradually increase the detection voltage to U0. The detection fre sequence is 0.01 Hz-0.02 Hz-0.05 Hz-0.1 Hz at each detection voltage.

Microscopical Physicochemical Measurement
In this experiment, we use the EAB parameter to characterize the XLPE mec properties. The dumbbell-shaped specimens are formed following the method de in 2.1 and tested using a tensile testing machine. Due to the sensitivity of EAB to face condition of specimens, three dumbbell-shaped samples are tested in eac group.
FTIR analysis evaluates the changes in the microstructure and physicochemic erties of the cable insulation, as well as cable stability. A Perkin-Elmer Cetus inst (Norwalk, CT) is used. First, 1 to 2 mg powder is ground from the inner layer samp sample powder is then fully mixed with KBr and placed into the mold. Finally, t required for FTIR analysis is produced by the pressing machine.
XRD is used to determine the crystalline structure changes in the XLPE ins The test equipment used here was RIGAKU. The test samples are all 20 mm × 20 m mm. The scanning range is 10° to 30° and scanning rate is 4°/min. The high-voltage FDS testing platform is composed of a high-voltage FDS detection system and cable sample, as shown in Figure 4. The cable core is connected to the output end of the detection system, and the cable ground wire is connected to the current detection end. The FDS is measured in the following order: start at 0.25 U 0 , then increase by intervals of 0.25 U 0 , gradually increase the detection voltage to U 0 . The detection frequency sequence is 0.01 Hz-0.02 Hz-0.05 Hz-0.1 Hz at each detection voltage. ment (FTIR, XRD, and mechanical test) are taken from the most strongly aging part o cable (inner slice). The aging time lasts 1536 h (64 days). The overall thermal aged cable is tested high-voltage FDS at the aging time points of 48 h, 96 h, 192 h, 384 h, 768 h, and 153 respectively, and the material is also collected for the mechanical properties test, F and XRD at the same time points.

Macroscopic High-Voltage FDS Measurement
The high-voltage FDS testing platform is composed of a high-voltage FDS detec system and cable sample, as shown in Figure 4. The cable core is connected to the ou end of the detection system, and the cable ground wire is connected to the current de tion end. The FDS is measured in the following order: start at 0.25 U0, then increas intervals of 0.25 U0, gradually increase the detection voltage to U0. The detection freque sequence is 0.01 Hz-0.02 Hz-0.05 Hz-0.1 Hz at each detection voltage.

Microscopical Physicochemical Measurement
In this experiment, we use the EAB parameter to characterize the XLPE mechan properties. The dumbbell-shaped specimens are formed following the method descr in 2.1 and tested using a tensile testing machine. Due to the sensitivity of EAB to the face condition of specimens, three dumbbell-shaped samples are tested in each a group.
FTIR analysis evaluates the changes in the microstructure and physicochemical p erties of the cable insulation, as well as cable stability. A Perkin-Elmer Cetus instrum (Norwalk, CT) is used. First, 1 to 2 mg powder is ground from the inner layer sample. sample powder is then fully mixed with KBr and placed into the mold. Finally, the required for FTIR analysis is produced by the pressing machine.
XRD is used to determine the crystalline structure changes in the XLPE insulat The test equipment used here was RIGAKU. The test samples are all 20 mm × 20 mm mm. The scanning range is 10° to 30° and scanning rate is 4°/min.

Microscopical Physicochemical Measurement
In this experiment, we use the EAB parameter to characterize the XLPE mechanical properties. The dumbbell-shaped specimens are formed following the method described in Section 2.1 and tested using a tensile testing machine. Due to the sensitivity of EAB to the surface condition of specimens, three dumbbell-shaped samples are tested in each aged group.
FTIR analysis evaluates the changes in the microstructure and physicochemical properties of the cable insulation, as well as cable stability. A Perkin-Elmer Cetus instrument (Norwalk, CT) is used. First, 1 to 2 mg powder is ground from the inner layer sample. The sample powder is then fully mixed with KBr and placed into the mold. Finally, the film required for FTIR analysis is produced by the pressing machine.
XRD is used to determine the crystalline structure changes in the XLPE insulation. The test equipment used here was RIGAKU. The test samples are all 20 mm × 20 mm × 0.6 mm. The scanning range is 10 • to 30 • and scanning rate is 4 • /min.

Test Results of Mechanical and Physicochemical Properties
The mean value of the EABs of three specimens and the sample variance versus aging time is shown in Figure 5; all data are from the most severely aged part (inner layer) of the cable insulation. Through the analysis of variance, it is found that there is no significant difference in the data of each group, meaning good consistency. From Figure 5, the test results of mechanical properties show that the overall change trend of XLPE material EAB has a good monotonic relationship with its aging degree. When the aging degree is mild (48-96 h), the change in EAB is relatively gentle. After aging for a period of time, the EAB begins to decrease, reaching a moderate stage of aging (192-384 h). At the severe aging stage (768-1536 h), due to the increase in chemical reaction rate, the decrease rate of EAB accelerates and rapidly decreases to the attention value (EAB 75%), and then the failure value (EAB 50%).
The mean value of the EABs of three specimens and the sample varianc time is shown in Figure 5; all data are from the most severely aged part ( the cable insulation. Through the analysis of variance, it is found that ther cant difference in the data of each group, meaning good consistency. From test results of mechanical properties show that the overall change trend of EAB has a good monotonic relationship with its aging degree. When the a mild (48 h-96 h), the change in EAB is relatively gentle. After aging for a p the EAB begins to decrease, reaching a moderate stage of aging (192 h-384 vere aging stage (768 h-1536 h), due to the increase in chemical reaction rat rate of EAB accelerates and rapidly decreases to the attention value (EAB 7 the failure value (EAB 50%). The FTIR spectra of cable insulation versus aging time are shown in F early aging stage (thermal aging 48 h-96 h), no significant change in the F XLPE is observed except for the slight fluctuation in 1020-1380 cm −1 . This fe explained by the fact that the antioxidant has not been consumed complet period. Additionally, the fluctuation of 1020-1380 cm −1 band should be rela sumption process of the antioxidants. These infrared absorption peaks ap 1078, 1159, and 1300 cm −1 , respectively [4]. After 192 h of thermal aging, aging stage, the antioxidant is gradually exhausted; it can be seen that a n peak appears at the wavenumber 1720 cm −1 , and the intensity of this absor creases with the increase in thermal aging time. This type of stretching vib cm −1 corresponds to carbonyl (-C=O), showing that carbonyl is produced polyethylene during thermal aging; the carbonyl absorption peak can be us acteristic peak characterizing thermal oxygen aging. At the same time, it ca from the spectrum that the intensity of the absorption band at the wavenum cm −1 region and the intensity of the absorption peak at 720 cm −1 and 1460 c icantly reduced, which is related to the increase in the carbonyl peak int emergence of the ether absorption peak at 1169 cm −1 (the vibration frequenc cm −1 ). A large amount of carbonyl production leads to the reduction in methyl groups (-CH, -CH2, and -CH3). The carbonyl absorption peak increa at wavenumber 1720 cm −1 after thermally aged for 784 h (i.e., at the late agin cating that the cable has undergone chain breaking and oxidation reacti period [8]. The FTIR spectra of cable insulation versus aging time are shown in Figure 6. At the early aging stage (thermal aging 48-96 h), no significant change in the FTIR spectra of XLPE is observed except for the slight fluctuation in 1020-1380 cm −1 . This feature could be explained by the fact that the antioxidant has not been consumed completely during this period. Additionally, the fluctuation of 1020-1380 cm −1 band should be related to the consumption process of the antioxidants. These infrared absorption peaks appear at about 1078, 1159, and 1300 cm −1 , respectively [4]. After 192 h of thermal aging, at the middle aging stage, the antioxidant is gradually exhausted; it can be seen that a new absorption peak appears at the wavenumber 1720 cm −1 , and the intensity of this absorption peak increases with the increase in thermal aging time. This type of stretching vibration at 1720 cm −1 corresponds to carbonyl (-C=O), showing that carbonyl is produced in crosslinked polyethylene during thermal aging; the carbonyl absorption peak can be used as the characteristic peak characterizing thermal oxygen aging. At the same time, it can also be seen from the spectrum that the intensity of the absorption band at the wavenumber 2750-3000 cm −1 region and the intensity of the absorption peak at 720 cm −1 and 1460 cm −1 are significantly reduced, which is related to the increase in the carbonyl peak intensity and the emergence of the ether absorption peak at 1169 cm −1 (the vibration frequency is 1000-1300 cm −1 ). A large amount of carbonyl production leads to the reduction in the number of methyl groups (-CH, -CH 2 , and -CH 3 ). The carbonyl absorption peak increased drastically at wavenumber 1720 cm −1 after thermally aged for 784 h (i.e., at the late aging stage), indicating that the cable has undergone chain breaking and oxidation reaction during this period [8].
The XRD spectra of XLPE insulation at different aging stages are shown in Figure 7. The spectra show two peaks at 21.7 • and 24 • , which are characteristic of the (110) and (200) lattice planes, respectively, in a crystal region of polyethylene. A peak at 19.5 • can be obtained through peak separation analysis using Gaussian function to the XRD spectra, which represents the amorphous region. The crystallinity can be obtained from the following relationship [23]:  The XRD spectra of XLPE insulation at different aging stages are The spectra show two peaks at 21.7° and 24°, which are characteristic of lattice planes, respectively, in a crystal region of polyethylene. A peak tained through peak separation analysis using Gaussian function to which represents the amorphous region. The crystallinity can be obta lowing relationship [23]: where WXLPE is the crystallinity, Ia, I110, and I200 are the diffraction inten (110), and (200) peaks, respectively. According to the specific changes in crystal diffraction peaks, t change in the position of the diffraction peaks during the whole aging that the basic crystal form of each aging stage is the same. The diffractio (110) and (200) peaks decrease with aging time after 384 h. In additio middle stage of thermal aging (after 384 h), serrated protrusions app spectrum; that is, the diffraction intensity increased or decreased near t With aging time increasing, the number of zigzag distortion protrusion peak distortion becomes stronger.  The XRD spectra of XLPE insulation at different aging stages are The spectra show two peaks at 21.7° and 24°, which are characteristic of lattice planes, respectively, in a crystal region of polyethylene. A peak tained through peak separation analysis using Gaussian function to which represents the amorphous region. The crystallinity can be obt lowing relationship [23]: where WXLPE is the crystallinity, Ia, I110, and I200 are the diffraction inten (110), and (200) peaks, respectively. According to the specific changes in crystal diffraction peaks, t change in the position of the diffraction peaks during the whole aging that the basic crystal form of each aging stage is the same. The diffractio (110) and (200) peaks decrease with aging time after 384 h. In additio middle stage of thermal aging (after 384 h), serrated protrusions ap spectrum; that is, the diffraction intensity increased or decreased near t With aging time increasing, the number of zigzag distortion protrusion peak distortion becomes stronger.

Analysis of the Physicochemical Properties
In the FTIR spectra of new cables and cable samples with differen grees, it was observed that the absorption peak at wavenumber 2010 c to thermal aging, and its strength is not affected by the degree of ther while the carbonyl absorption peak is the opposite. Therefore, it is suit characteristic 2010 cm −1 peak as a reference peak. The carbonyl index is of carbonyl absorption peak intensity at wavenumber 1720 cm −1 to abs sity at wavenumber 2010 cm −1 (A1720/A2010) [24]. According to the specific changes in crystal diffraction peaks, there is no obvious change in the position of the diffraction peaks during the whole aging period, indicating that the basic crystal form of each aging stage is the same. The diffraction intensities of the (110) and (200) peaks decrease with aging time after 384 h. In addition, at the end of the middle stage of thermal aging (after 384 h), serrated protrusions appeared in the XRD spectrum; that is, the diffraction intensity increased or decreased near the diffraction peak. With aging time increasing, the number of zigzag distortion protrusions increases and the peak distortion becomes stronger.

Analysis of the Physicochemical Properties
In the FTIR spectra of new cables and cable samples with different on-site aging degrees, it was observed that the absorption peak at wavenumber 2010 cm −1 is not sensitive to thermal aging, and its strength is not affected by the degree of thermal oxygen aging, while the carbonyl absorption peak is the opposite. Therefore, it is suitable to calibrate the characteristic 2010 cm −1 peak as a reference peak. The carbonyl index is defined as the ratio of carbonyl absorption peak intensity at wavenumber 1720 cm −1 to absorption peak intensity at wavenumber 2010 cm −1 (A1720/A2010) [24].
Carbonyl Index(CI) = Absorption at 1720cm −1 (the maximum of carbonyl peak) Absorption at 2010cm −1 The CI change in the cable's inner insulation during thermal aging is shown in Table 1. The CI can be used to characterize the material thermal aging degree. As shown in Table 1, CI exhibits almost no change from 48 to 96 h (the early aging stage). During 192-384 h (middle aging stage), CI begins to increase due to the consumption of antioxidants and the production of polar products. At the late aging stage (after 384 h), CI increases sharply; this rapid rise in CI can be attributed to the radicals generated by the chain scission of the polymer in the presence of oxygen, meaning that this process is accompanied by chain breaking and the formation of a large number of polar oxidation products. Additionally, the radicals caused by chain breaking could lead to the formation of pairs of alkoxy radicals. The reactions of alkoxy radicals could result in the formation of more polar groups, such as carbonyls, aldehydes, and ketones [24], making the CI increase further. In order to further analyze the spectral lines, the half-peak width (FWHM) of the crystallization peak and crystallinity during thermal aging are also calculated. The FWHM of the two crystal peaks and crystallinity at each aging stage are also shown in Table 1. According to the Table 1, the FWHM fluctuates firstly at the early aging stage. Then, with the increase in aging time, during 192-384 h, the (110) and (200) peaks start to broaden. The serrated protrusion and broadening of the half-peak width indicates the distortion of the XRD diffraction spectra and the appearance of zigzag protrusion, whereas the broadening of peak shape indicates that dislocation defects have been initiated in the crystal zone, and that some crystal atoms deviate from the original position. At this stage, although the crystallinity does not strongly decrease, the crystal region has been destroyed slightly. When the damage degree of crystal zone is low, the crystallinity does not decrease greatly (48-384 h). Whereas with the aging degree deepening, at the late aging stage, FWHM of (110) and (200) peaks broaden rapidly and the serrated protrusion density increases. With the increase in dislocation defect density in the material, the damage degree of crystal region increases, resulting in the decrease in crystallinity (after 784 h) [25].

High-Voltage FDS Results
The high-voltage FDS of unaged cable sample is shown in Figure 8. In Figure 8, the maximum tanδ of the unaged cable at each detection voltage level can only reach about 1.0 × 10 −3 (U 0 , 0.01 Hz), and tan δ does not increase or decrease regularly with the frequency change. Moreover, the curves show cluster shape and do not separate in the frequency domain. According to the microscopical physicochemical test results, the aging deg divided into three phases: the early, middle, and late aging stages. The high-volta curves of the cable at the early aging stage (48 h-92 h) are shown in Figure 9a,b. T curve shapes of the unaged and early aging stage cable are similar, but the tan δ show an increasing trend. In addition, the tan δ of the early aging stage cable incr the detection frequency decreases, indicating the slope of the tan δ-f curves increa pared to the unaged cable. However, at this aging time point, the cable is still at th of early aging, the increase in dielectric loss is not large, and there is no regular v in the change in voltage level. Correspondingly, the performance changes are not o As shown in Table 1, the EAB, CI, and crystal structure exhibit almost no change. The high-voltage FDS curves of the cable at the middle aging stage (192 h-38 shown in Figures 10a and 7, respectively. According to Figure 7, the tan δ value co to increase with the aging degree. In particular, starting from a higher voltage lev the tan δ-f curves gradually increase as the detection voltage rises at each frequenc meaning the FDS curves start to show a layered phenomenon. Correspondingly, ing to the results in 3.1, at this aging time point, the mechanical properties begin t a declining trend, the polar products increase, and dislocation defects begin to ap the crystal region. According to the microscopical physicochemical test results, the aging degrees are divided into three phases: the early, middle, and late aging stages. The high-voltage FDS curves of the cable at the early aging stage (48-92 h) are shown in Figure 9a,b. The FDS curve shapes of the unaged and early aging stage cable are similar, but the tan δ values show an increasing trend. In addition, the tan δ of the early aging stage cable increases as the detection frequency decreases, indicating the slope of the tan δ-f curves increase compared to the unaged cable. However, at this aging time point, the cable is still at the stage of early aging, the increase in dielectric loss is not large, and there is no regular variation in the change in voltage level. Correspondingly, the performance changes are not obvious. As shown in Table 1, the EAB, CI, and crystal structure exhibit almost no change. According to the microscopical physicochemical test results, the aging degree divided into three phases: the early, middle, and late aging stages. The high-voltage curves of the cable at the early aging stage (48 h-92 h) are shown in Figure 9a,b. The curve shapes of the unaged and early aging stage cable are similar, but the tan δ v show an increasing trend. In addition, the tan δ of the early aging stage cable increas the detection frequency decreases, indicating the slope of the tan δ-f curves increase pared to the unaged cable. However, at this aging time point, the cable is still at the of early aging, the increase in dielectric loss is not large, and there is no regular vari in the change in voltage level. Correspondingly, the performance changes are not obv As shown in Table 1, the EAB, CI, and crystal structure exhibit almost no change. The high-voltage FDS curves of the cable at the middle aging stage (192 h-384 h shown in Figures 10a and 7, respectively. According to Figure 7, the tan δ value conti to increase with the aging degree. In particular, starting from a higher voltage level the tan δ-f curves gradually increase as the detection voltage rises at each frequency p meaning the FDS curves start to show a layered phenomenon. Correspondingly, ac ing to the results in 3.1, at this aging time point, the mechanical properties begin to s a declining trend, the polar products increase, and dislocation defects begin to appe the crystal region. The high-voltage FDS curves of the cable at the middle aging stage (192-384 h) are shown in Figures 7 and 10a, respectively. According to Figure 7, the tan δ value continues to increase with the aging degree. In particular, starting from a higher voltage level (U 0 ), the tan δ-f curves gradually increase as the detection voltage rises at each frequency point, meaning the FDS curves start to show a layered phenomenon. Correspondingly, according to the results in Section 3.1, at this aging time point, the mechanical properties begin to show a declining trend, the polar products increase, and dislocation defects begin to appear in the crystal region. According to the microscopical physicochemical test results, the aging degree divided into three phases: the early, middle, and late aging stages. The high-voltage curves of the cable at the early aging stage (48 h-92 h) are shown in Figure 9a,b. The curve shapes of the unaged and early aging stage cable are similar, but the tan δ v show an increasing trend. In addition, the tan δ of the early aging stage cable increas the detection frequency decreases, indicating the slope of the tan δ-f curves increase pared to the unaged cable. However, at this aging time point, the cable is still at the of early aging, the increase in dielectric loss is not large, and there is no regular vari in the change in voltage level. Correspondingly, the performance changes are not obv As shown in Table 1, the EAB, CI, and crystal structure exhibit almost no change. The high-voltage FDS curves of the cable at the middle aging stage (192 h-384 h shown in Figures 10a and 7, respectively. According to Figure 7, the tan δ value conti to increase with the aging degree. In particular, starting from a higher voltage level the tan δ-f curves gradually increase as the detection voltage rises at each frequency p meaning the FDS curves start to show a layered phenomenon. Correspondingly, acc ing to the results in 3.1, at this aging time point, the mechanical properties begin to s a declining trend, the polar products increase, and dislocation defects begin to appe the crystal region. The high-voltage FDS curves of the cable at the late aging stage (after 384 h) are shown in Figure 11a,b. At the late aging stage, the cable tan δ value continues to increase, up to about a maximum of 180 × 10 −3 (U 0 , 0.01 Hz). The layered phenomenon of the tan δ-f curves becomes increasingly obvious at each detection voltage level. At this stage, on the one hand, the decreased rate of EAB accelerates and rapidly decreases to the attention value (EAB 75%) then the failure value (EAB 50%). On the other hand, chain breaking and oxidation reaction produce more polar groups, and high-density defects in the crystal zone of material lead to a decrease in crystallinity.
Polymers 2022, 14, x FOR PEER REVIEW up to about a maximum of 180 × 10 −3 (U0, 0.01 Hz). The layered phenomenon of the f curves becomes increasingly obvious at each detection voltage level. At this stage, one hand, the decreased rate of EAB accelerates and rapidly decreases to the att value (EAB 75%) then the failure value (EAB 50%). On the other hand, chain breakin oxidation reaction produce more polar groups, and high-density defects in the c zone of material lead to a decrease in crystallinity. According to [15,16], the high-voltage FDS of water-tree-aged cable also increa the detection voltage rises. In addition, the high-voltage FDS of the water-tree-aged also exhibits the hysteretic effect, i.e., when a higher voltage is applied between the s at the same low voltage level, the loss levels from a second measurement sweep same voltage level) are higher than those in the previous sweep. The previous an second FDS curves of low voltage level (0.25 U0) before and after a higher voltage the late aging stage (thermally aged 1536 h) are shown in Figure 12. According to F 12, the lower voltage detection curves are crossed in frequency domain. This mean even at the late stage, thermal aging does not cause the hysteresis effect. Therefo water treeing and thermal aging can be distinguished by hysteresis phenomenon.

Correlation Analysis of High-Voltage Dielectric and Physicochemical Propert
The XLPE molecular structure is shown in the Figure 13. As seen in Figure 13, is composed of crystalline region and amorphous region. A main chain can pass th several crystalline and amorphous regions, and the chain is rigidly arranged in p with the crystalline region, and curled and irregularly arranged in the amorphous r The dipolar groups interact with each other via the backbone of the chain, as also ind schematically in Figure 13. This interaction process may produce the material loss. the reversal of any one dipole must react on relatively distant dipoles through the According to [15,16], the high-voltage FDS of water-tree-aged cable also increases as the detection voltage rises. In addition, the high-voltage FDS of the water-tree-aged cable also exhibits the hysteretic effect, i.e., when a higher voltage is applied between the sweeps at the same low voltage level, the loss levels from a second measurement sweep (at the same voltage level) are higher than those in the previous sweep. The previous and the second FDS curves of low voltage level (0.25 U 0 ) before and after a higher voltage (U 0 ) at the late aging stage (thermally aged 1536 h) are shown in Figure 12. According to Figure 12, the lower voltage detection curves are crossed in frequency domain. This means that, even at the late stage, thermal aging does not cause the hysteresis effect. Therefore, the water treeing and thermal aging can be distinguished by hysteresis phenomenon.
Polymers 2022, 14, x FOR PEER REVIEW up to about a maximum of 180 × 10 −3 (U0, 0.01 Hz). The layered phenomenon of th f curves becomes increasingly obvious at each detection voltage level. At this stage one hand, the decreased rate of EAB accelerates and rapidly decreases to the at value (EAB 75%) then the failure value (EAB 50%). On the other hand, chain break oxidation reaction produce more polar groups, and high-density defects in the zone of material lead to a decrease in crystallinity. According to [15,16], the high-voltage FDS of water-tree-aged cable also incre the detection voltage rises. In addition, the high-voltage FDS of the water-tree-age also exhibits the hysteretic effect, i.e., when a higher voltage is applied between the at the same low voltage level, the loss levels from a second measurement sweep same voltage level) are higher than those in the previous sweep. The previous second FDS curves of low voltage level (0.25 U0) before and after a higher voltage the late aging stage (thermally aged 1536 h) are shown in Figure 12. According to 12, the lower voltage detection curves are crossed in frequency domain. This mea even at the late stage, thermal aging does not cause the hysteresis effect. Theref water treeing and thermal aging can be distinguished by hysteresis phenomenon.

Correlation Analysis of High-Voltage Dielectric and Physicochemical Prope
The XLPE molecular structure is shown in the Figure 13. As seen in Figure 13 is composed of crystalline region and amorphous region. A main chain can pass t several crystalline and amorphous regions, and the chain is rigidly arranged in with the crystalline region, and curled and irregularly arranged in the amorphous The dipolar groups interact with each other via the backbone of the chain, as also in schematically in Figure 13. This interaction process may produce the material los the reversal of any one dipole must react on relatively distant dipoles through th chain linking them, and it is intuitively plausible to assume that the resulting ener constant is independent of the rate of reversal, but dependent on the number of r dipoles and their reversal amplitude [26].

Correlation Analysis of High-Voltage Dielectric and Physicochemical Properties
The XLPE molecular structure is shown in the Figure 13. As seen in Figure 13, XLPE is composed of crystalline region and amorphous region. A main chain can pass through several crystalline and amorphous regions, and the chain is rigidly arranged in parallel with the crystalline region, and curled and irregularly arranged in the amorphous region. The dipolar groups interact with each other via the backbone of the chain, as also indicated schematically in Figure 13. This interaction process may produce the material loss. Here, the reversal of any one dipole must react on relatively distant dipoles through the rigid chain linking them, and it is intuitively plausible to assume that the resulting energy loss constant is independent of the rate of reversal, but dependent on the number of reversed dipoles and their reversal amplitude [26].
Polymers 2022, 14, x FOR PEER REVIEW Figure 13. The arrangement of XLPE molecular structure and its main chain.
From the perspective of polarization, the polarization strength P of the mater be considered as a response caused by the electric field strength E, that is [27]: The non-aging insulation is non-polar, the dipole density is low, so the dielect of the cable changes little with frequency and detection voltage. In the early stages ing, antioxidants are not consumed. At this stage, on the one hand, the oxidation r will lead to chain breaking (thermal oxidative cracking) of polyethylene molecules. while, on the other hand, due to the presence of antioxidants, small molecules wil crosslinked, thus, there are few oxidation products and low polarization strength. fore, there is no obvious change in the CI. Correspondingly, during this period, alt the dielectric loss and curve slope increase slightly, the change is not obvious w increase in detection voltage and frequency. With the extension of aging time, a dants are gradually depleted. At the middle aging stage, the CI starts to increase cating that the XLPE molecular chain starts to break and the oxidation reaction be occur. From this stage, the increase in polar matter on the one hand leads to the in in dipole density in the material, which helps to improve the polarization intens the other hand, chain breaking makes dipole reversal easier. Thus, during the mid ing period, from the higher detection voltage, the polarization strength increases, s the material loss. At the late thermal ageing stage, the CI increases drastically, sh more oxidation polar products. During this period, the dipole density is higher a dipole rotation is less affected by the rigid main chain.
Furthermore, according to the XRD results, at the beginning of the middle s thermal aging, serrated protrusions appear in the XRD spectrum, that is, the diff intensity increases or decreases near the diffraction peak. In addition, from Table half-peak width of XLPE in the middle stage of thermal aging is widened. The ab sults show that dislocation defects appear in the material at this stage, and some atoms deviate from the original position. At this stage, although the crystallinity d decrease greatly, the crystal region has been destroyed. Thus, the rigidity of th chain of XLPE molecule begins to decrease, the probability of dipole reversion inc and the reversal amplitude also increases. Especially when the external electric large, the loss of the cable begins to increase significantly, that is, the cable appears ination. At the late stage of thermal aging, dislocation defect density gradually in drastically, and the crystallinity of XLPE begin to decrease significantly, which me increase in the amorphous region. At this aging time point, the reversion probab the dipole continues to increase and the inversion angle is larger. There is a stable s cation phenomenon in FDS spectrum and the loss increases.
According to Figure 12, even at the late stage, thermal aging does not cause th teresis effect. Some researchers believe that the water tree insulation consists of sm ter-filled voids separated by crazing zones. When increasing the test voltage, M From the perspective of polarization, the polarization strength P of the material can be considered as a response caused by the electric field strength E, that is [27]: The non-aging insulation is non-polar, the dipole density is low, so the dielectric loss of the cable changes little with frequency and detection voltage. In the early stages of aging, antioxidants are not consumed. At this stage, on the one hand, the oxidation reaction will lead to chain breaking (thermal oxidative cracking) of polyethylene molecules. Meanwhile, on the other hand, due to the presence of antioxidants, small molecules will be re-crosslinked, thus, there are few oxidation products and low polarization strength. Therefore, there is no obvious change in the CI. Correspondingly, during this period, although the dielectric loss and curve slope increase slightly, the change is not obvious with the increase in detection voltage and frequency. With the extension of aging time, antioxidants are gradually depleted. At the middle aging stage, the CI starts to increases, indicating that the XLPE molecular chain starts to break and the oxidation reaction begins to occur. From this stage, the increase in polar matter on the one hand leads to the increase in dipole density in the material, which helps to improve the polarization intensity. On the other hand, chain breaking makes dipole reversal easier. Thus, during the middle aging period, from the higher detection voltage, the polarization strength increases, so does the material loss. At the late thermal ageing stage, the CI increases drastically, showing more oxidation polar products. During this period, the dipole density is higher and the dipole rotation is less affected by the rigid main chain.
Furthermore, according to the XRD results, at the beginning of the middle stage of thermal aging, serrated protrusions appear in the XRD spectrum, that is, the diffraction intensity increases or decreases near the diffraction peak. In addition, from Table 1, the half-peak width of XLPE in the middle stage of thermal aging is widened. The above results show that dislocation defects appear in the material at this stage, and some crystal atoms deviate from the original position. At this stage, although the crystallinity does not decrease greatly, the crystal region has been destroyed. Thus, the rigidity of the main chain of XLPE molecule begins to decrease, the probability of dipole reversion increases, and the reversal amplitude also increases. Especially when the external electric field is large, the loss of the cable begins to increase significantly, that is, the cable appears delamination. At the late stage of thermal aging, dislocation defect density gradually increases drastically, and the crystallinity of XLPE begin to decrease significantly, which means the increase in the amorphous region. At this aging time point, the reversion probability of the dipole continues to increase and the inversion angle is larger. There is a stable stratification phenomenon in FDS spectrum and the loss increases.
According to Figure 12, even at the late stage, thermal aging does not cause the hysteresis effect. Some researchers believe that the water tree insulation consists of small water-filled voids separated by crazing zones. When increasing the test voltage, Maxwell mechanical stresses will cause water to penetrate the crazing zones, thus forming electric contact between the elongated water droplets. The electric field at the tip of these conductive channels is enhanced, resulting the dielectric losses increase [28]. However, the opening and closing processes of the channel are gradual [29]. Therefore, in a certain time interval, the water tree channel is not completely closed in the second low voltage detection, resulting in the second tan δ value is greater than the first tan δ value. As previously analyzed, the dielectric property change during thermal aging is mainly caused by polarization and dipole reverse, which is different from the change mechanism of water tree aging, thus there is no hysteresis effect. Therefore, we consider that the hysteresis effect can be used to distinguish the water tree aging from thermal aging.

Discussion on Diagnosis Method
According to the comparison of Figures 8-11, that the tan δ-f curves increase with the rising detection voltage for the cable with a certain aging degree. Additionally, the curve presents a layered characteristic in the frequency domain, meaning at any detection frequency point, tan δ (U 0 ) > tan δ (0.75 U 0 ) > tan δ (0.5 U 0 ) > tan δ (0.25 U 0 ). The ratio of dielectric loss at higher voltage to that at lower voltage must be greater than 1. Therefore, the layered characteristic of the high-voltage FDS curves can be extracted as the quantity to characterize the change in XLPE cable FDS. The layering degree (L) is defined as the dielectric loss ratio mean value of each adjacent detection voltage level at each frequency point: where L 1 , L 2 , and L 3 are the layering degrees of FDS curves between different voltage levels, n is the number of detection frequency points, and f k is the detection frequency.
The L matrix of high-voltage dielectric FDS curves of thermal aging cable at various aging stages is shown in Table 2. According to the layered characteristics comparison and analysis of the thermally aging cable, it can be seen that the layering coefficient L 1 , L 2 , and L 3 are not all greater than 1 when the cable is not aged and with weaker aging degree. While with the deepening of aging severity, the appearance of polar substances and the change in crystal structure lead to the molecular polarization increase. Therefore, the FDS curve tan δ-f rises with the detection voltage level increase, resulting in the layered phenomenon of FDS curves, so the layered degree L 1 , L 2 , and L 3 are all greater than 1. Therefore, the layering coefficient matrix of high-voltage FDS curve can be used to diagnose the cable insulation thermal aging.
From the change in the high-voltage FDS with aging time (Figures 8-11), the slope (K) and integral (S) of the curve change distinctly with the aging degree. Moreover, the detection voltage also has a significant effect on these two parameters. The FDS test voltage influence on K and S at different aging stages are shown in Figure 14. According to Figure 14, K and S increase with aging time. Especially, when the detection voltage is higher (U 0 ), the change is more obvious. Therefore, K and S under U 0 detection voltage can be used as characteristic values to diagnose aging degree. The changes in K and S under U0 detection voltage (KU0/SU0) are also shown in 2. In the early aging stage (48-96 h), the KU0 and SU0 values almost do not change; middle aging stage (192-384 h), KU0 and SU0 begin to increase; after 384 h, KU0 and S sharply. These trends are consistent with the change in CI, FWHM, and crystallin Table 1), EAB (in Figure 5). Therefore, we consider that KU0 and SU0 are suitable cha istics to diagnose the aging degree.

Conclusions
In this study, the high-voltage FDS (0.25 U0-U0, 0.01 Hz-0.1 Hz) of an XLPE with different aging degrees and the physical, chemical, and mechanical properties insulation materials are tested. In addition, the relationships between these factors a explored. On this basis, the diagnosis and evaluation method of cable thermal aging on high-voltage FDS is discussed.
(1) The high-voltage FDS of the cables with a certain degree of thermal aging pr layered characteristics, which is different from low-voltage FDS. The experimental r suggest that the layering degree L of high-voltage FDS is greater than 1, thus the la degree matrix can be used as the judgment basis for cable aging.
(2) According to the FTIR and XRD results, it is indicated that the increase in groups (carbonyls, aldehydes, and ketones, etc.) and crystal regions are produced b The changes in K and S under U 0 detection voltage (K U0 /S U0 ) are also shown in Table 2. In the early aging stage (48-96 h), the K U0 and S U0 values almost do not change; in the middle aging stage (192-384 h), K U0 and S U0 begin to increase; after 384 h, K U0 and S U0 rise sharply. These trends are consistent with the change in CI, FWHM, and crystallinity (in Table 1), EAB (in Figure 5). Therefore, we consider that K U0 and S U0 are suitable characteristics to diagnose the aging degree.

Conclusions
In this study, the high-voltage FDS (0.25 U 0 -U 0 , 0.01 Hz-0.1 Hz) of an XLPE cable with different aging degrees and the physical, chemical, and mechanical properties of the insulation materials are tested. In addition, the relationships between these factors are also explored. On this basis, the diagnosis and evaluation method of cable thermal aging based on high-voltage FDS is discussed.
(1) The high-voltage FDS of the cables with a certain degree of thermal aging presents layered characteristics, which is different from low-voltage FDS. The experimental results suggest that the layering degree L of high-voltage FDS is greater than 1, thus the layering degree matrix can be used as the judgment basis for cable aging.
(2) According to the FTIR and XRD results, it is indicated that the increase in polar groups (carbonyls, aldehydes, and ketones, etc.) and crystal regions are produced by molecular bond breakages, causing an increase in the polarization intensity. The above factors drive the layered characteristic; with the increase in aging degree, the layered phenomenon of the aging cables is more obvious. This is due to the sharp increase in polar groups and the decrease in crystallinity, which reduces the rigidity of molecular main chain and makes it easier for dipoles to reverse.
(3) The changes in the tan δ-f curve slope and integral (K U0 /S U0 ) are very sensitive to thermal aging effects, and they also change consistently with the micro-physical characteristics change: CI, FWHM, crystallinity and elongation at break. Therefore, they can be considered as useful diagnostic parameters for estimating the degree of XLPE thermal aging deterioration.